A magnetic recording medium is widely used as a recording tape, a video tape, a floppy disk, etc. A magnetic recording medium is fundamentally composed of a nonmagnetic support having formed thereon a magnetic layer composed of a ferromagnetic powder dispersed in a binder.
A magnetic recording medium is required to be excellent in various characteristics such as electromagnetic characteristics, running durability, running property, etc. That is, in an audio tape for recording and reproducing music, a higher original sound reproducing faculty is required. Also, a video tape is required to be excellent in electromagnetic characteristics such as original image reproducing, etc.
Furthermore, a magnetic recording medium is required to have such excellent electromagnetic characteristics, and at the same time have good running durability as described above. For obtaining good running durability, the function of abrasives has an important role. That is, the abrasives contained in the magnetic layer are distributed all over the magnetic layer, but parts thereof exist at the surface of the magnetic layer. Consequently, when the magnetic recording medium runs and comes in contact with parts of the running route, such as a magnetic head, etc., the abrasives existing at the surface of the magnetic layer give a polishing effect at the contact surface between the surface of the magnetic layer and the magnetic head, etc. Accordingly, by containing abrasives in the magnetic layer, the running durability of the magnetic recording medium can be improved. However, the abrasives existing at the surface of the magnetic layer are only a part of the abrasives added to the magnetic layer. The abrasives remaining in the magnetic layer may make it difficult to obtain a sufficiently excellent running durability.
For example, when the amount of abrasives is increased for improving the running durability of a magnetic recording medium, the content of a ferromagnetic powder is reduced, and; further, when abrasives having large particle sizes are used, the abrasives are liable to properly project over the surface of the magnetic layer. This results in deterioration of the electromagnetic characteristics. Also, the use of the abovedescribed abrasives having large particle sizes together with abrasives having small particle sizes has been described in JP-A-57-162129 and JP-A-58-85931 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but by the foregoing methods, sufficient electromagnetic characteristics have not been obtained.
For example, JP-A-58-85931discloses that the magnetic layer contains two kinds of abrasives each having a different mean particle size, e.g, an abrasive having a mean particle size of from 0.05 to 0.30 .mu.m, and an abrasive having 0.5 to 15 .mu.m. However, even by the combination of such abrasives, it is impossible to improve C/N while preventing the increase of drop-out, and; further, it is impossible to prevent the reduction of the output.
Additionally, the use of abrasives having a Mohs' hardness of more than 9.0 is described, e.g., in JP-B-47-18572, JP-B-50-3936, JP-B-51-2801, JP-B-58-45088 (the term "JP-B" as used herein means an "examined published Japanese patent application") and JP-A-59-227026, but by the foregoing disclosures, the polishing faculty of the magnetic recording medium is improved but the good surface property of the magnetic layer cannot be obtained. For example, in the technique disclosed by JP-A-59-227026, the surface property of about 25 nm only is obtained. On the other hand, by subjecting the magnetic recording medium to a surface smoothening treatment using metal rolls, the surface property of the magnetic layer is improved but the durability of the magnetic layer is not. Also, by using a metal magnetic powder having fine particle sizes, C/N is improved but the durability of the magnetic layer is not.
Recently, in a video tape using a metal (or alloy) ferromagnetic powder for increasing the high recording density, the reduction of the recording width and the reduction of the thickness of the magnetic layer has been practiced. However, this resulted in a narrowing of the width of a magnetic recording head whereby the durability of the magnetic head was reduced. Thus, a hard material such as silicon nitride, aluminum nitride, etc., has been used at the head portion of the magnetic head, as disclosed in JP-A-2-210606. The use of such a magnetic head causes problems in that the video tape is scratched by the magnetic head, the occurrence of drop-out is increased, and the output of the magnetic recording medium is reduced. Further, the increase of the polishing faculty for the magnetic head encounters the trouble of reducing the output of the video tape.
For improving C/N, it is necessary to increase the packing density of the ferromagnetic powder, and to achieve this purpose, it has been desired to keep the polishing faculty of the magnetic layer with a lower amount of abrasives. Also, for improving C/N, super smoothening of the surface of the magnetic layer and fining of the size of a ferromagnetic powder are effective, but in this case, problems occur; namely, the surface of the magnetic layer is scratched by a hard magnetic head, and the durability of the magnetic recording medium is reduced.